Electromagnetic Interference shields, also referred to as EMI shields, are widely used in direct-view electronic displays to suppress unwanted radiation of electromagnetic energy or electromagnetic interference. Displays such as Cathode Ray Tube (CRT) and plasma display panels can produce undesirable EMI emissions that generally must be reduced to levels dictated by various regulatory agencies such as the Federal Communications Commission (FCC). One known type of EMI shield is constructed, for example, by bonding an electrically conductive mesh to a transparent substrate such as glass or plastic. Such EMI shields are described, for example, in U.S. Pat. Nos. 6,399,879; 6,492,587; 6,049,037; and 6,086,979.
A major potential drawback common to these types of construction is the general loss of imaging light, or display brightness, caused by the EMI shield. In particular, a metallic EMI shield that passes visible light while efficiently blocking RF energy generally uses a fine mesh (e.g. about 50 μm or finer) composed of solid conductors with an array of gaps between the conductors that are small compared to the wavelength of radio frequency (RF) energy (i.e. several mm to a few cm in typical displays). Conventional conductive materials such as copper or aluminum are not transparent to visible light and therefore block some of the display light, resulting in loss of brightness. Transparent conductors such as Indium Tin Oxide (ITO) can provide conductivity without impairing visible transmission, but the conductivity of this material is generally relatively low and it is generally very expensive compared to common metals. The conductor pattern should also be sufficiently small so that it is not visible to the unaided eye.
Commercial EMI shield products have been created that can provide effective EMI shielding within regulatory limits, but these products generally have low visible light transmission ranging from about 40% to about 60%. Improving this transmission would not only provide a brighter display, but may permit the display to operate at lower power, hence more efficiently, and with less heat generation and/or with longer life.